Crystal Structure and Comparative Crystal Chemistry of AI2M~(OH)12(C03) · 3H20, a New Mineral from the Hydrotalcite-Manasseite Group A

Crystallography,Repons, Vol. 41, No.6, 1996, pp. 972-981. Transltnedfrom Kristallograjiya, VoL 41, No.6, 1996, pp. 1024-1034. @ Original Russian Text Copyright 1996 by Aralccheeva, Pushcharovskii, Rastwetaeva, Atencio, Luhman.

Crystal Structure and Comparative Crystal Chemistry of AI2M~(OH)12(C03) · 3H20, a New Mineral from the Hydrotalcite-Manasseite Group A. v. Arakcheeva*, D. Yu. Pushcharovskii**, R. K. Rastsvetaeva***, D. Atencio****, and G. U. Lubman* Baikov Institute of Metallurgy, Russian Academy of Sciences, Leninskii pro 49, Moscow, 117334 Russia * Moscow State University, Moscow, 119899 Russia ** *** Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pro 59, Moscow, //7333 Russia **** University of Sao Paulo, Sao Paulo, Brazil Received April 9, 1996

. 3H~O from Ihe Abstract-The crystal structure of a new mineral of the composition AI2Mg4(OH)12(C03) hydrotalcite-manasseite group has been determined by X-ray structure analysis. The parameters of Ihe hexagonal unit cell are a =5.283(3), c = 15.150(9) A; sp. gr. P6 2m. The structure was refined over 52 cryslallograph- ically nonequivalent reflections up to R =0.039. The hydrogen atoms of hydroxyl groups are localized. The new mineral differs from otherrepresentatives oHhis group by the complete order of all the atoms, which is rctkl'!ed in a lower structure symmetry. The variety of minerals within the group is considered and interpreled in lerms of polytypism, the atomic order in the structure of sublattices, and isomorphous substitutions.

INTRODUCTION Later on, one more mineral-chlormagaluminite (Mg, Fe)4A12(OH)dCI, (C03)o.sh 2H:O-w~ Depending on the composition of the octahedral included into the hydrotalcite-manasseite ..ubgroup. brucite-type cationic layers composing the structures of The hypothetical structure model of this mineral admits a very large group of natural and synthetic compounds the ordered arrangement of Al atoms in the (Mg, Fe, (the so-called dihydroxides), one can distinguish three AI3+, A1)-octahedra of a brucite layer [5]. The mineral of subgroups in which the cations are (a) Mg2+ + composition II, recently approved by the Comission on (b) Mg2++ Fe3+, and (c) Mg2++ Cr3+.The octahedral New Minerals and Mineral Names (CNMMN) of the environment of the cations is formed by hydroxyl International Mineralogical Association (no. 92-028 (OHt ions, whereas the interlayer voids are filled with [6]), should also be related to this subgroup. although (C03)2- anions and H20 molecules. The two modifica- the detailed mineralogical and structural data for this tions characteristic of these compounds are considered mineral have not been published. In relation to this, the as polytype 2H and 3R modifications with different unit mineral from the Jacupiranga alkaline-ultrabasic car- cell parameters a and c (in the hexagonal setting): 3.1 bonatite massif (Sao Paulo, Brazil), first described as and 15.6 Afor the former and 3.1 and 23.4 Afor the lat- manasseite [7] but, in fact, having a composition con- ter polytype. sistent with the data for a new representative of the sub- In the subgroup of (Mg, AI)-hydroxides of the com- group of the dihydroxides under consideration, is of positions Mg6AI2(OH)16(C03). 4H20 (I), the 2H poly- great interest, and the determination of its structure was type is manasseite and the 3R polytype is hydrotalcite; the main purpose of this study. in the subgroup of the (Mg, Fe) hydroxides having similar compositions, the 2H polytype is sjogrenite and the 3R polytype is pyroaurite. The corresponding minerals EXPERIMENTAL in the subgroup of (Mg, Cr)-hydroxides are stichtite The mineral under consideration was discovered in and barbertonite. The above compositions of natural the form of bright orange-red 2-6-mm large grains in and synthetic hydrotalcite [1-3] differ from the compo- dolomites [6]. The crystals had hexagonal trapezoidal sition established in the X-ray structure determination faceting, with prismatic faces and basal pinacoidal . 3H20 (II) [4]. of this mineral-Mg4AI2(OH)12(C03) faces, parallel to which perfect cleavage was observed. Hydrotalcite formulas 1 and II have different numbers X-ray single-crystal studies (a SYNTEX automatic dif- of C03 groups, the same numbers of (Mg + AI) cations, and different values of the Mg :Al ratio (3 : 1 for 1 and fractometer, sp. gr. PI, MoKa radiation) were per- 2 : 1 for II). formed on a O.025-mm-thick crystalline platelet witha

@ 1063-7745/96/4106-0972$10.00 1996 MAMK Hayr<:a /Interperiodica Publishing CRYSTAL STRUCTURE AND COMPARATIVE CRYSTAL CHEMISTRY 973

Table1. Characteristics of the minerals of the hydrotalcite-manasseite subgroup according to different sources Sp. gr. or Chemical formula Unit cell parameters, References Mineral lattice type A

HydrotaIcite AI2M&(OH)16C03 . 4H20 (I) R3m a = 3.102, C = 23.404 [2] AI2M&(OH)16C03 . 4H20 (I) R a = 6.142, C = 46.24 [8]

AI2M~(OH)12C03 . 3H20 (II) R3m a =3.054, C =22.81 [4] Manasseite AI2M&(OH)16C03 . 4H20 (I) P63/mmc a=3.1,c=15.5 [1] AI2M&(OH)16C03 . 4H20 (I) H a = 6.13, c = 15.37 [9] Chlonnagaluminite AI2(Mg, FeMOH)12 . (Cl, (C03)O.5h . 2H2O P63/mcm a =5.29, c =15.46 [5] maximumdimension in the plane of 0.625 mm. The this space group up to the reliability factor R =0.12. twelvemost intense reference reflections provided the The model structure consisted of two brucite-type lay- establishmentof a hexagonal unit cell with the lattice ers perpendicular to the 63 axes, each of them being parameters a' = 3.050(1) and c = 15.150(9) A, which, built by three (Mg, Al)-octahedra (layer l' in Fig. 2a). togetherwith the data on the chemical composition, The 0 and C atoms are located between the layers at the indicatedthat the crystal belonged to the hydrotalcite- same height z (the 0 net in Fig. 2a). In addition to the manasseitesubgroup (Table 1).As is seen from Table 1, 6raxis, the brucite layers were also related by the plane thebasis parameters of the hexagonal unit cells can be m normal to this axis and passing through the (C, 0) chosen differently depending on the presence or nets and by the coordinate plane c absent in the initial absenceof a small number of weak reflections indicat- symmetry group. The interlayer nets related by the ingan increase in the minimum translation vector a': 6raxis and the c plane also had half-filled positions of (i)a = a' = 3.05 A, (ii) a = 2a' = 6.1 A, or (iii) a = C and 0 atoms of the C03 groups and H20 molecules. a' J3 = 5.28 A. There is another, fourth, possible vari- The 50% occupancy of the C-position lying at the intersection of the 2 and 63 axes with the three 0 atoms ant of the translation, (iv) a = 2a' J3 = 10.56 A (Fig. 1). bound to it and also at the intersection of three 0 atoms To avoid any possible loss of experimental data, the from the H20 molecules followed from the chemical basic set of X-ray reflections was obtained for the unit composition of compound II and was confirmed by the cell with the maximum parameter, a = 2a'J3 = electron-density values on the F-synthesis for the 10.566 A, within half of the Ewald sphere (sinS/A. = model obtained. The identical interlayer nets allowed 0.81A-l). Altogether, 262 reflections with I > 2cr(I) us to describe the model within the sp. gr. P63/mcm, wererecorded. The analysis of their intensities indi- which was inconsistent with the presence of weak cated the diffraction class 6/ mmm. Upon averaging, the reflections not obeying the extinction rule imposed by experimental set contained 52 crystallographically independent reflections; the reliability factor for the 2a'J3 equivalent reflections was R =0.059 (over 39 groups of a = reflections). The absence of reflections with odd h and k indices indicated a parameter half as large, a = 5.283 A. a=a'J3 The attempt to ,.pass to the unit cell with the parameter a = 2a' = 6.01 A with the aid the matrix (2/31/30, -1/3 1/30,001) resulted in the loss of seven reflections; the remaining reflections had either even h or even k indices. This indicated the presence of a pseudotranslation a' =3.05Aalong the [210] direction in the unit cell with the parameters a = 5.283(3) and c = 15.150(9) A. Weperformed a complete structure determination withinthis unit cell. All of the calculations were per- formedusing the AREN program complex [10] taking into account anomalous atomic scattering. Structure amplitudes were calculated from the intensities with dueregard for the polarization and Lorentz factors. b = 2a'J3 The regular extinctions in the intensities of the (0001)reflections with I =2n + 1 within the diffraction Fig. 1. Relationship between the a vectors of the hexagonal class6/mmm indicated the space group P6322. A struc- unit cells in the structures of various minerals of the ture model was determined by direct methods within dihydroxide group.

CRYSTALLOORAPHY REPORTS Vol. 41 No.6 1996 974 ARAKCHEEV A et ai. two threefold axes, in accordance with chemical formula n. A high value of the thermal parameter of the C atom (> 10 A2) indicated possible splitting of its position. The F-synthesis provided the localization of the peak lying at a distance of 0.3 A from the initial position of the C atom (and from the m plane) along the z-axis. Thus, we placed this atom into this position with 50% occupancy. The refinement of the split C-position reduced the reliability factor down to R =0.055 and the thermal parameter down to 6.0 A2. The H atoms of the hydroxyl groups were localized from the difference synthesis. We failed to localize the H atoms of the H20 molecules, probably because of the high mobility characteristic of free water molecules (which was seenfrom the high values of their thermal parameters). The refinement of the positional parameters of H atoms reduced the R-factor down to 0.047, whereas the allowance for the anisotropy of thermal vibrations of Al and Mg atoms and the anisotropic extinction (E =0.000(222) reduced it even more--down to R =0.039. The difference electron-density maps showed no valuable peaks. The positional and thermal parameters of the atoms are listed in Table 2, and the interatomic distances are given in Table 3. It should be emphasized that the crystal was characterized by a weak scattering power, the number of experimental reflections was rather small, and, there- (a) (b) fore, the accuracy of the coordinate and interatomic distance determination was relatively low. .C o 0 0 H20 .,1/2C CD1/20 e 1/2H20 STRUCTURE DESCRIPTION Fig. 2. The scheme of (a) disordered (-0--1'-0--1'-0--) and (b) ordered (-2-1-3-1-2-) arrangement of atoms in the The crystal structure determined is built by three layer structural elements in the hexagonal unit cell with the types of layer elements (1, 2, and 3 in Figs. 2b and 3) parameter a 5.28 A. Notation: a brucite layer is = a'J3 "" perpendicular to the 6 -axis and bonded to one another denoted by 1 and l' for the ordered and disordered variants of octahedra filling, respectively; the interlayer net with the by hydrogen bonds (Fig. 3). Layer 1 is of a brucitetype statistical distribution of 0 atoms is denoted by the number and has the composition [AIMg2(OH)6]' it consists of 0; the carbonate-containing net and the H20-net are denoted AI- and Mg-octahedra, and the 0(1) and 0(2) atomsof by the numbers 2 and 3, respectively. hydroxyl groups form a close-packing motif in two O-nets (with three atoms Rer unit cell in each net) and the plane c. The inconsistency of the symmetry ele- are spaced 2.91 and 3.19 A from one another. Layer2 ments of the model obtained and the experimentally is a carbonate net of the composition [C03] consisting observed regular extinctions and also the ambiguous of three 0(3) atoms displaced from the close-packing structure of the interlayer nets containing the COr positions toward the C atom located on one of the three threefold axes in a way such that four 0(3)-0(3) dis- group and three H20 molecules led to the sp. gr. P6 2m tances between the C02- anions are 3.53 A and two in which the two nearest nets in the planes m could be 0(3)-0(3) distances inside these anions are 2.23 A. independent and could contain COrgroups (the first Layer 3 is a net of three H20 molecules whose 0 atoms one) and triads of H20 molecules (the second one) are also arranged by the motif of a close packed layer (Fig. 2b). This assumption was confirmed in the course with four H2O-H20 distances equal to 2.79 A and two of the structure determination and refinement. The distances equal to 3.68 A. The common feature of all (0001) extinctions are explained by the equal scattering three structural elements is oxygen nets built by the power of the (C03) and (3H20) nets spaced by a half- same number of atoms arranged by the close-packing translation along the z-axis. motif. Then, the structure model (Fig. 2b) was refined Three layer structural elements along the hexagonal within the group P6 2m up to R =0.06 in the isotropic axis form the sequence -1-2-1-3-1-. The two closest approximation. The Al and Mg atoms were located on layers 1 are related by the symmetry planes m passing

CRYSTALLOGRAPHY REPORTS Vol. 41 No.6 1996 CRYSTAL STRUCTURE AND COMPARATIVE CRYSTAL CHEMISTRY 975

Table 2. Positional and thermal (A2) parameters of atoms in essential characteristic not only of this structure but theAI2M~(OH)12C03 . 3H20 structure also of other structures of the compounds of this group. Atom xIa ylb zlc Beq or Biso (*) Mg 1/3 2/3 0.243(3) 1.8(1) RESULTS AND DISCUSSION AI 0 0 0.259(3) 2.6(1) We established a new, rather interesting ordered 0.021(8) C 0 0 6.3(9)* arrangement of the interlayer C02- and H20-nets in the 0(1) 0 0.651(2) 0.325(8) 2.9(2) structure, which was never observed earlier in any 0(2) 0.318(2) 0 0.190(1) 2.7(2)* related minerals of the dihydroxide group. According 0(3) 0.244(3) 0 0 4.3(6)* to the known structure data for the compounds of this H2O 0.598(5) 0 1/2 6.5(8)* group calculated only for the hexagonal unit cell with the parameter a = 3.054 A [4], the interlayer nets are H(I) 0.35(3) 0.35(3) 0.398(9) 3.0* characterized by a statistic distribution of H20 mole- H(2) 0 0.36(3) 0.125(9) 3.0* cules and C03 groups, which is, in fact, the superposi- tion of nets 2 and 3 observed in this study (the 0 layer through the nets of layers 2 and 3. Each 0 atom of the in Fig. 2a) projected onto the unit cell with the param- interlayer net is located between two OH groups of the eter a = 3.054 A (the 0 layer in Fig. 4a). For the chlo- neighboring brucite layers and fonns hydrogen rine-containing variety of manasseite, chlormagalu- O-H...O.. .H-O bonds providing layer interactions. minite (Table 1), it was possible (with due regard for Ifone uses the notation of close-packed layers and weak reflections) to choose a unit cell with the param- neglects the small deviations from close packing, the eter a =5.29 A, but the structure of the interatomic lay- sequence of O-nets in the structure can be written as ers was not established. By analogy with the minerals AIA~IBIB3BI (Fig. 3), where the numerical subscript of the pyroaurite-sjogrenite subgroup, which, accord- corresponds to the structural element of the layer. The ing to [1], are almost identical (their structures differ position of 0 atoms in nets 2 and 3 is uniquely deter- only in the cationic composition), it was always mined by the position of the corresponding hydroxyl assumed that, in the minerals of the manasseite-hydro- groupsfrom layer 2. The neighboring interlayer O-nets talcite subgroup, the H20 molecules and C03 groups 2 and 3 are displaced with respect to one another, in are randomly distributed over the interlayer nets. More- accordance with the close-packing law, as are the O(H) over, the structures of the minerals of this group (con- nets of layer 1. It should be emphasized that each 0 sidered in detail elsewhere [1]) were always described atomof the interlayer nets between 2 and 3 is an accep- within the unit cell with the parameter a = 3.1 A, tor for two hydrogen bonds with two OH-groups, despite the fact that in some cases weak reflections cor- which gives a ratio of 2 : 1 between the number of responding to the parameters a = 5.37 and a = 6.2 A OH-groups and the number of 0 atoms in the interlayer [11-13] were observed. It was noted [1] that the chem- nets irrespectively of the net compositions. It seems ical analysis yields an k[3+: AP+ratio of either 1 : 3 or that thereare no other interactionsbetween the struc- 1 : 2 and only sometimes 1 : 5 for the statistical distri- ture layers, which is confirmed by a large distance bution ofthese cations in brucite layers. betweennets 2 and 3 and the 0 atoms from layer 1: 2.88 and 2.65 A, in comparison with the distance 2.04 A The crystallochemical analysis of the dioxide crys- betweenthe O-nets in layer 1. The positions of H20 tal structures based on our results and the data reported moleculeswith respect to Al and Mg atoms are equiva- in [1-9, 11-15] allowed us to draw some new conclu- lent, whereas the C atoms are located on the same sions about possible variations in their composition, threefoldaxis with AI atoms, which, we believe, is an structures, and the unit-cell parameters. The analysis

Table3. Interatomic distances (A) and angles (deg) in H-bonds in the AI2M~(OH)12C03 . 4H20 structure Mg-octahedron AI-octahedron COr group Mg-O(2) 1.97(2) x 3 AI-O(2) 1.98(3) x 3 C-O(3) 1.32 x 3 -0(1) 2.12(3) x 3 -0(1) 2.09(2) x 3 -C 0.6(1) (2.05) (2.04) H-bonds H2O-H20 2.79(1) H(I)-O(I) 1.0(1) H(2)-O(2) 1.0(1) H20 1.6(1) 0(32) 1.99(1) ... LO(I)-H(I) ... H20 168(7) LO(I)-H(2) 0(3) 149(7)

CRYSTALLOGRAPHY REPORTS Vol. 41 No.6 1996 976 ARAKCHEEVA et at. ct of one brucite layer and two adjacent interlayer nets: each of which belongs to this structural unit for onll 50%. One of these interlayer nets contains only COl groups, whereas the other consists of H20 molecules, l t {~ (5) The brucite layer has the compositios 0(3~\ ; •••.; -A2}2 [(Al, Mg)i0H)2y], where the values y = 1, 3,4, and 12 Jb;I\ I correspond to the composition of the hexagonal uns cell with the parameters a :::::3.05, 5.3, 6.1, and 10.6 A, 0(2) -AI} 1 respectively (Fig. 5a). (6) Both brucite layers and interlayer nets are bound ~!WWi-BI by hydrogen O-H··.O(A)···H-Q bonds in which theI accepters are the atoms of the interlayer nets: either Q or additional A anions (e.g., CI in chlormagaluminite), o Therefore, the ratio of the total number of acceptcs (0, A) atoms to the number of OH groups is 1 : 2 and with due regard for assumption (5), the number oJ (0, A) atoms equals the number of (AI, Mg) cations ill the brucite layer. Since OH groups form layers by the close-packing motif, the interlayer (0, A) nets are al~ arranged by a motif close to the close-packing motif.

CHEMICAL COMPOSITION AND THE VARIATIONS IN THE a PARAMETER OF THE HEXAGONAL UNIT CELL As has been noted above, the dihydroxide group 15 often described by chemical formula I with the ratio Of the number of atoms from the interlayer nets to the number of OH groups being 7 : 16 instead of 1 : 2. This difference can be explained by three factors: (1) some hydroxyl groups do not participate in hydrogen bondl- ing because there is no acceptor atom in the interlaycr Fig. 3. The (100) projection of the Al2Mg4(OH)!2

CRYSTALLOGRAPHY REPORTS Vol. 41 No.6 1996 CRYSTAL STRUCTURE AND COMPARATIVE CRYSTAL CHEMISTRY 977 grains with the pseudoparameters 7.6 and 7.8-7.9 Ain o 2 polycrystalline specimens can be found in [1, 11]. Proceeding from electrical neutrality of the compounds under consideration, hydrogen bonding, and the possible presence of additional monovalent A- anions, one can write the chemical formula of minerals of the hydrotalcite-manasseite series in the form

AlxMg(y-x)(OHhiC03)(x_d)I2A~ .H20(y -3xl2+dl2).(III)

In the general case, the A-anions can be located in any interlayer nets. Let us analyze formula m for the composition of the chosen structural unit in hexagonal unit cells with the above-mentioned four different values of the parameter a (Figs. 1, 5). 3 (i) a = a' 3.05 A (Fig. 4). The composition of minerals of the ""hydrotalcite-manasseite series admits the presence of defect-free brucite layers (with the statistical filling of octahedra) and interlayer nets with H20 (a) (b) molecules in the structure with the minimum parameter. In the nets containing C03 groups in the absence of oxygen defects, the positions of C atoms on one Fig. 4. A structural unit in the hexagonal unit cell with the parameter a = a' = 3.01 A. For notation, see Fig. 2. of the threefold axes should be filled with C-atoms (a) Totally disordered variant (--0-1'--0-); all the atoms in only partly. For the case under consideration, the num- the interlayer O-type nets have the occupancy coefficient ber of atoms in formula III is (AI + Mg) = y = 1, 1/6; (b) ordered alternation of interlayer C03- and H20-nets because there is only one cation position in a brucite by the law (-2-1'-3-); the occupancies of all the positions layer.The nonnegativity condition for the stoichiomet- of the interlayer nets are 1/3. ric coefficients in the transformed formula III, AlxMg(l-x)(OHMC03)(x-d)/2Ad. H20(l-3x/2+d/2),yields If a compounds has a carbonate group but has no theinequality 0 ::;d::;x ::;1. The total number of accep- A anions (d = 0 in IV), then Al : Mg = 1 : 3, which is tor atoms in the interlayer nets and the number of cat- quite consistent with possible cation ordering in the ionsin a brucite layer are equal to unity; therefore, the brucite layer for the unit cell under consideration (layer distribution of additional A anions and the filling of 1 in Fig. 6) and coincides with the common value of octahedra with the cations of different valences can this ratio for these cations. Nevertheless, in this case, it onlybe statistical. follows from IV that the number of H20 molecules A multiple increase in the parameter a can be differs by 0.5 from the given values in each structural explainedby the ordering of the Al and Mg cations in a unit. A small number of additional monovalent A- brucitelayer or by the ordered arrangement of atoms in anions makes this discrepancy less obvious, but, upon aCOr containing interlayer net. Below, we consider in increasing the number of trivalent cations, one can detailthe effect of both these factors on the variations also obtain instead of the Al : Mg = 1 : 3 ratio the ratio of the parameter a. For the sake of simplicity, Fig. 5 (1 + d) : (3 - d). To the compositions I often given showsan idealized arrangement of atoms fully occu- for hydrotalcite and manasseite (Table 1) there corre- pying their positions according to° the close-packing sponds a vacancy instead of an acceptor atom in the motif(Fig. 5b). interlayer net (Ad= 00.5)' and formula III takes the form AlMg3(OH)g(C03)o.5Do.5 2H20 = (ii) a = 2a' 6.1 A (Fig. 6). The sum of the cations ""Mg2+) 1/2(AI2Mg6(OH)16(C03) . 4H20) = 1/2 (I). The vacant in III is (AI3++ = Y = 4. The number of acceptor position in the carbonate net (0 in Fig. 6) admits the atomsin each interlayer net is (0 + A) 4. Therefore, = entrance of additional atoms into the structure. This thecarbonate net should contain one C03 group com- was also confirmed in [3]. plemented with one (H20) or A acceptor, or else it should be completely substituted by four such addi- We have found no reliable data on the compounds described by formula V.At the same time, a somewhat tional atoms. Therefore, there is either 0.5 (C03) per simplified compound without A anions and with a min- structuralunit or else there are no such groups at all, imum unit cell with a = a' was mentioned as a possible andformula III takes one of two forms: composition elsewhere [1, 14]. AI(l d)Mg(3 d)( H)g( C03)o.0d . H20(2.5 d) (IV) + - ° - (iii) a = a' J3 5.3 A. For this case, formula III has or "" (AI3+ + Mg2+) = Y = 3 with the number of acceptor atoms in each interlayer net (0 + A) also equal to 3. The

CRYSTALLOORAPHY REPORTS Vol. 41 No.6 1996 978 ARAKCHEEVA et al.

carbonate net can contain only one C03 group. It is this variant of the composition that is described in this structural study (Fig. 2b). Formula III takes the form AI(I+d)Mg(2-d)(OHMC03)o.0d . H20(1.5-d)' Without additional A atoms (at d = 0), III = (1/2) (II). TheAl: Mg = I : 2 ratio corresponds to a possible cationic ordering in the brucite layer and ordering of C atoms in the carbonate net (Fig. 2b). At this value of the ratio of the number of differently charged cations observed by different authors? the thickness of one structural unit is minimal (~7.6 A) and indicates the absence of additional acceptor atoms or vacancies. (a) In principle, A atoms can enter the structurc only if they substitute water molecules, which should be a' 2a' accompanied by an increase in the number of trivalent cations in comparison with their number at an AI : Mg = a'J3 I : 2 ratio and a simultaneous reduction of the number ,~' ' of H20 molecules. If one takes this into account, the 2a'J3" D' '0. formula of chlormagaluminite [5] with the proper number of water molecules and the parameter (/ = 5.29 A '0., 0 o o o 0' should be written as Mg3Fe3+AI2(OH)I~CO~C] .2H20 " and not as (Mg, Fe2+)4A1iOH)12«CO,)" ,. CII~ . 2H20 o , D" (Table 1). The main difference reduces to the degree of ' iron-ion oxidation and the separation of some additional Cl-ions from the C03 group. (b) (iv) a = 2a'J3 ~ 10.6 A (Fig. 7). In this case, the number of cations in the brucite layer in fonnula III is Fig. 5. Showing (a) the orientation of the reference basis (AP+ + Mg2+) y 12, i.e., equal to the number of vectors of possible hexagonal unit cells with respect to the = = brucite layer with disordered cations, and (b) an idealized acceptor atoms (0 + A) in each interlayer nel. If lhe car- close-packed interlayer net. bonate net contains only four C03 groups. thcn the structural unit reduces to the unit cell with the parameter a ~ 5.3 Aconsideredin (Ui). Formally, another possible composition of thc structural unit 2C03 + 3(H20, A) (2 in Fig. 7a) described by formula III = A1(3+d)Mg(9_d)(OHb(COdI0d . H20(7.5-d) for a carbonate-containing nct can be reduced to formula IV considered in (ii). although at the given a parameter the order of cations with different valences and C atoms can be different (cf. Figs. 6 and 7a). All the other remarks about the composition coincide with those given in (U). To the composition of the carbonate-containing net 2C03 + 6(H20,A) (Fig. 7b) there corresponds the structural-unit formula III =AI(2+d)Mg(1O~diOH)~IC00d' H20(9-d)' Without allowance for additional anions in (b) the interatomic nets, the ratio of the numbers of cations of different valences is I : 5. The possible formation of similar structures is confirmed by the data given in [1, 11]. Figure 7b illustrates possible cation ordering in a brucite layer for this case. And, finally, one more possible composition of the: interlayer net containing C03 groups is C03 + 7(H20, Fig. 6. Schematic of (a) possible order of cations in the bru- A). The structural unit here is described by the chemical cite-type layer and (b) the carbonate-containing nets in the formula III = Al(1 +d)Mg(1l-d)(OHb(C03)o.0d . hexagonal unit cell with the parameter a =2a' 6.1 A; Al : '" Figure 7c illustrates the structure of the Mg = I : 3. A small square denotes a position containing H20(10.5 _d)' either H20-molecules or vacancies. carbonate-containing net and possible ordering of cat-

CRYSTALLOGRAPHY REPORTS Vol. 41 No.6 1996 CRYSTAL STRUCTURE AND COMPARATNE CRYSTAL CHEMISTRY 979

(a) (b) (c)

Fig. 7. The scheme of possible ordering of cations in a brucite layer and a carbonate-containing net in the hexagonal cell with the parameter a = 2a' J3 10.56 A for the ratio of cations of different valences (a) AI: Mg = 1 : 3, (b) 1 : 5, and (c) 1 : 11. For notation, see Fig. 6. ions in a brucite layer. In this case, the Al : Mg ratio polytype modifications, each of which can uniquely be becomes Al : Mg = 1 : 11. We have not found any indi- described by a sequence of the symmetry operations cations of such a ratio in any publications, but a close indicated above (m and 2). The simplest variants mm... ratio (1 : 12) was reported in [1]. and 22... correspond to the well-known dihydrate mod- A similar analysis can also be performed for larger ifications 2H and 3R [1]. The symbols H and R indicate hexagonal cells. It is possible to substantiate other hexagonal and rhombohedral unit cells, whereas the ratios of cations with different valences. It should be numbers indicate the number of layer structural units. noted that any of the above considered (and not consid- As follows from the previous consideration, for the sta- ered) unit cells can readily be projected onto the mini- tistical distribution of the interlayer nets 2 and 3 (i.e., mum unit cell with the statistical distribution of cations their complete identity), the polytype modification 3R in a brucite layer and the statistical filling of the inter- should have the parameter c = 3c' (",,23A) reported in layer nets (Fig. 4a). In fact, this is confirmed by some [1, 2, 4]. To this case there corresponds the sequence structural studies. AoAIBIBoBICICOC)A)Aoof close-packed O-nets, where o denotes the statistically filled interlayer net. If nets 2 and 3 alternate, then the parameter c of the modification POLYTYPE MODIFICATIONS AND VARIATIONS is doubled in accordance with the sequence IN THE PARAMETER c OF THE HEXAGONAL A0)BiB~)C)C2C)AIA0IB)B2BIC)C3CIAIA2' and its UNIT CELL symbol :should be written as 6R. This parameter c = The chosen layer structural unit of the thickness c' 6c' = 46.24 A for the hydrotalcite structure is given in "" 7.6 Acan be described in terms of close packed O-nets [8]. as A~)BIB3 (see, e.g., Fig. 3), where the subscripts The generalized results of the theoretical crystal- indicate which structural elements these nets belong to. lochemical analysis of some possible compounds of the The combination of these structural units sharing the dihydroxide group with due regard for possible varia- boundary interlayer nets 2 and 3 along the c-axis can be tions in their composition, the parameters of the hexag- described as either A0)B)B~)AIA2 or onal unit cell, various polytype modifications, and the A1fi)BIB~1 CIC2. degrees of order are given in Table 4. These results can In the former case, the nearest brucite layers along also be extended to other dihydroxide subgroups. the direction of their alternation (AIB) and BIAI) are Along with the well-known polytypes 2H and 3R, we related by the symmetry plane m passing though the also considered 4H polytypes with the parameter c = interlayer net In the latter case, they are rotated with 4c' 31 mentioned in [1, 14]. In the derivation ofthe B3' "" A respect to one another (AIB) and BICI) by 1800 about space groups and possible degrees of order, it was taken the axis 2 passing through the 0 atoms of the interlayer into account that the above-described 1800-rotation of net (B3) normal to its plane. These two ways of layer neighboring brucite layers in the unit cells with a = combination predetermine the existence of various a'J3 and a = 2a' J3 lead to changes in the position

CRYSTALLOGRAPHY REPORTS Vol. 41 No.6 1996 980 ARAKCHEEV A et al.

Table 4. Characteristics of hydrotalcite-manasseite-type compounds obtained from the theoretical crystallochemical analysis , >, Unit cell parameters and space groups for different types of ordering -s:;::o in structures Co..::!0 o ~Generalized chemical formula and its particu- ..r:::u ....t;:: ordered alterna- ordering of cat- lar cases at d = 0 (a, b) 'O:e fully disor- -0 dered fully ordered tion of interlayer ions in brucite- ]S (-2-1-3-1-) nets type layers 0 (-0- 1'-0- 1') E>,Co (-2-1'-3-1'-) (-0-1-0-1-) en.?:- 3+ 2+ mm... a =d, C =2c' l)a=2a',c=2c' a =a', C l)a=2d,c=2c' M(1 +d) M(3_d)(OHMC03)0.sAd' H2O(2,5-d) = 2c' P63/mmc P6m2 P6m2 P63/mmc (*) (*) 2) a = 2a'J3, 2) a = 2a'J3, c=2c' c=2c' P62m P63/mcm (a) M23+M62+(OH)16(C03), 5H2O, 22... a = a', c = 3c' a = 2d, c = 6c' a = d, c = 6c' a = 2a', c = 3c' R3m R3m R3m R3m (**) (**) 3+ 2+ m2... a = d, C =4c' a = 2d, C= 4c' a = a', c = 4c' a = 2a', c = 4c' (b) M2 M6 (OH)16(C03)TJ' 4H20 (*, **) P63/mmc P6m2 P63/mmc P63/mmc

mm ... a = d, C = 2c' a = a', c a = d J3 , C = 2c' = 2c' a=dJ3,c=2c' Mg + d)Mg _diOHMC03)0.sAd H20(1.5-d)(***) P63/mmc P62m P6m2 P63/mcm (***) (a) M;+ M;+ (OH)dC03), 3HzO(**' @) 22... a =d, c = 3c' - a = d, c = 6c' - R3m R3m (**) m2... a =d, C =4c' - a=d,c=4c' - P63/mmc P63/mmc 3+ 2+ mm ... a=a',c=2c' a =a', c= 2c' M(2+d) M(10-d~(OHb(C03)Ad' H2O(9-d) a=2dJ3,c=2c a = 2a'J3, c = 2c' P63/mmc P6m2 P63/mcm 3+ 2+ P62m (a) M2 MlO (OHb(C03). 9HzO 22... a = d, c = 3c' - a =a', c= 6c' - 3+ 2+ R3m (b) M 2 M 10 (OH)24(C03)D3 . 6H2O R3m m2... a = a', c = 4c' - a = d, c = 4c' '- 3+ 2+ P63/mmc P63/mmc M(1 +d) M(1l_dj(OH)24(C03)o,sAd' H20(10.5-d) 3+ 2+ (a) M2 M22 (OH)4S(C03) . 21H2O 3+ 2+ (b) M2 M22 (OH)4S(C03)D9. 12H2O

Note: For minerals of the hydrotalcite-manasseite subgroup, a' = 3.05-3.10 and c' = 7.6-7.8 A; *, **, and *** are the characteristics for manasseite, hydrotalcite, and chlormagaluminite, respectively, according to various sources; @ is the new mineral considered in this study; (-1-) and (-1') denote the ordered and disordered variants of the arrangement of cations of different valences in a brucite layer; (-0-), (-2-), and (-3-) are the interlayer nets statistically filled with 0 atoms without the localization of C atoms, carbonate- containing nets, and H20 nets, respectively. multiplicities for differently charged cations. There- subgroup. As a result of the analysis performed, we fore, in this case no variants of the structure with the substantiated theoretically, for the first time, the exper- ordered arrangement of cations can exist. imentally observed integer ratios for di- and trivalent cations and established the reasons for the variations in CONCLUSION the parameters of their hexagonal lattice associated with changes in the chemical composition. The main The study of a new mineral with the ordered struc- conclusions drawn reduce to the following. ture from the hydrotalcite-manasseite subgroup allowed us to revise and systematize the known struc- (1) The unit cell with the parameter a ::z 3.05 A tural characteristics of various representatives of this describes only a fully disordered variant of the struc-

CRYSTALLOGRAPHY REPORTS Vol. 41 No.6 1996 CRYSTAL STRUCTURE AND COMPARATIVE CRYSTAL CHEMISTRY 981 ture,both with respect to the anion and cation positions. H20. Compounds with a higher number of water mole- Withinthis unit cell, one cannot explain the experimen- cules were reported in [15]. tally obtained ratios of the constituent elements in the As is seen from Table 4, the formation of the mineral chemical formulas of the minerals of the dihydroxide modifications or varieties in the group of compounds group. under consideration is directly related to the most 3+ 2+ important crystallochemical phenomena such as iso- (2) The compound of the M2 M6 (OH)16C03' morphism, polytypism, and structural ordering. As a 4H20 type with the ratio W+ : M2+ = 1 : 3 can be result of these processes, the chemical composition of described within the hexagonal unit cell with the the compounds changes both qualitatively and quanti- parametera 6.1 A. The structures of these minerals tatively; the space groups and the unit-cell metrics also havevacancies"" in the interlayer nets, which provides change. The minerals of this group have different thepossibility for some additional ions to enter into names either because of their different chemical com- thesenets with the simultaneous substitution of some positions (chlormagaluminite) or because they are divalentcations by cations of higher valence. The W+ : described by the R or P lattices (hydrotalcite and W+ = 1 : 3 ratio follows from the composition and manasseite, respectively). Both of these features make structure of the carbonate-containing interlayer net in the mineral studied above very close to manasseite. At thisunit cell and can be attained as a result of filling of the same time, the mineral differs from manasseite by octahedra in a layer. the value of the parameter a of the hexagonal unit cell and by the space group, which determine the maximum (3) The compounds of the M23+M42+ (OH)12C03' possible order of its structure. 3H20 type with the W+ : M2+ = 1 : 2 ratio can have a hexagonal unit cell with a 5.3 A. Some additional ACKNOWLEDGMENTS "" anionscan enter these structures only due to the substi- This study was supported by the Russian Founda- tution of water molecules under the condition of an tion for Basic Research (project nos. 95-05-15699 and increasing number of trivalent cations. The W+ : M2+ = 96-05-64- 381). 1: 2 ratio follows from the composition and structure ofthe interlayer C03 net and admits the ordered distri- butionof cations of different valences over the octahe- REFERENCES draof a brucite layer. 1. Taylor, H.F.W., Mineral. Mag., 1973, vol. 39, no. 304, (4) The compounds with the cation ratio W+ : M2+ = p.377. 1: 5 and their ordered arrangement can crystallize in 2. Allman, R.,Am. Mineral., 1968, vol. 53, p. 1057. thehexagonalunit cell with a 10.56 A. The same unit 3. Miyata, S., Clays Clay Miner., 1983, vol. 31, no. 4, cellcan also be formed in the""structures with the ratio p.305. of cations of different valences W+ : M2+ = 1 : 3, but 4. Allman, R. and Jepsen, H.P., Neues Jahrb. Mineral. witha somewhat different scheme of their ordering in Monatsch., 1969, no. 12, p. 544. comparison with the unit cell characterized by the 5. Kashaev, A.A., Feoktistov, G.D., and Petrova, S.V., Zap. parameter a 6.1 A. The same unit cell allows the Vses. Mineral. O-va., 1982, vol. 111, no. 1, p. 121. "" ordered cation arrangement at the W+ : M2+ = 1 : 11 6. Mandarino, J.A., Mineral. Rec., 1994, vol. 25, p. 315. ratio. 7. Menezes, L.A.D. and Martins, J.M., Mineral. Rec., 1984, vol. 15, p. 261. (5) The integer ratio of W+ and M2+ follows from theordered structure of the carbonate-containing inter- 8. Tables of JCPDS, Oslo: Mineral. Museum, no. 14-191. layernet and manifests itself in a regular increase of the 9. Tables of JCPDS, 1957, no. 14-525. parametera of the hexagonal unit cell. The most often 10. Andrianov, V.I., Kristaliografiya, 1987, vol. 32, no. 1, encountered ratios of the cations of different valences, p.228. W+ : M2+= 1 : 3 or 1 : 2, correspond to the minimum 11. Gastuche, M.C., Brown, G., and Mortland, M.A., Clay incrementin periodicity. Miner., 1967, vol. 7, p. 177. (6) The constant value of the above ratios in differ- 12. Aminoff, G. and Broome, B., Handl. Kungl. Vetensk., 9, 1930, vol. 3, no. 5, p. 23. ent specimens indirectly indicates that the equally 13. Frondel, C., Am. Mineral., 1941, vol. 26, p. 295. probableinterlayer carbonate-containing nets and H20- netsin the dihydroxide structures are independent. An 14. Taylor, H.F.W., Mineral. Mag., 1969, vol. 37, p. 338. increase of this ratio towards higher M2+ contents 15. Moore, P.B., Lithos, 1971, no. 4, p. 213. shouldbe accompanied by an increase in the H20 con- tentand the approach of the composition to Mg(OH)2 . Translated by L. Man

CRYSTALLOGRAPHYREPORTS Vol. 41 No. 6 1996